When an integrated circuit chip is being handled there is a danger that an electrostatic discharge (ESD) onto one or more of the chip's pins will damage the chip's circuitry. One way to reduce the possibility that an ESD will damage a chip is to incorporate “clamping circuitry” within the chip. The clamping circuitry is typically made up of a “voltage clamp” at each of the chip's pins (or “terminals”), with the exception of the ground pin(s). The clamp associated with a given pin is designed to divert ESD charge from the pin to ground or to the positive power supply. To prevent the voltage between the positive power supply and ground from becoming large enough to damage internal circuitry, a “power supply shunt” is placed between the positive supply and ground. The clamps and shunts operate together to prevent the voltage between any pair of terminals within the chip from exceeding a maximum allowed voltage.
When an integrated circuit chip is not being handled and is powered-up for operation, a termination voltage (Vtt) is typically applied to circuitry at each of the chip's input and output pins. The termination voltage is a power supply voltage associated with a terminator. The terminator, typically a resistive element with resistance equal to the characteristic impedance of the transmission line connected to the pin, is used to “absorb” signal reflections from impedance discontinuities in the transmission line at a transmitting pin, and to prevent reflections from signals at a receiving pin. Accordingly, the clamp and shunt elements must be able to safely withstand the termination voltage that is applied when the chip is in operation.